Devices for providing mechanical ventilation to assist patient respiration are well known. Such ventilator devices have been used to help patients with such ailments as severe lung disease, chest wall disease, neuromuscular disease and other diseases of respiratory control. Generally, a ventilator provides air or oxygen-enriched air to a patient at pressures that are higher during inspiration and lower during expiration.
Several types of ventilator devices exist. These types include bi-level ventilators, proportional assist ventilators and servo-controlled ventilators. Each type of ventilator utilizes different methods for assisting with patient respiration and achieves different goals.
Bi-level ventilators provide the simplest level of support. These ventilators supply a mask pressure P(t) which is higher by an amplitude A from an initial pressure P0 when respiratory airflow f(t) is inspiratory, f(t)>0, than when respiratory airflow is expiratory, f(t)≦0.
P(t) = P0 + Af(t) > 0(inspiration)P(t) = P0otherwise(expiration)Thus, these devices supply a fixed degree of support A. However, they do not guarantee any particular ventilation when, for example, the patient's efforts are inadequate.
Proportional assist ventilators represent an attempt to provide support more closely in phase with the patient's respiratory efforts. Proportional assist ventilators provide mask pressure as follows:
P(t) = P0 + R f(t) + ELC ∫ f(t)dtf(t) > 0(inspiration)P(t) = P0 + R f(t)otherwise(expiration)where R is a substantial fraction of the patient's airway resistance, and ELC is a substantial fraction of the patient's lung plus chest wall elastance. So long as there is no leak, this provides support much more closely in phase with a patient's respiratory efforts. However, again, in the case of a patient's efforts being inadequate, for example, due to reduced chemoreflex control of breathing during sleep, there is no guaranteed minimum ventilation.
One method of ensuring an adequate degree of ventilatory support is to use a servo ventilator, which adjusts the degree of support A to servo-control instantaneous ventilation V(t) to equal a target ventilation VTGT:
P(t) = P0 + Af(t) > 0, or time sincethe start of the lastinhalation > TMAXP(t) = 0otherwisewhere:A = G ∫ ( V(t) − VTGT ) dtV(t) = 0.5 abs ( f(t) )AMIN < A < AMAX.In this ventilator, V(t) is one half the absolute value of the respiratory airflow, G is the gain of the integral servo-controller, a value of 0.3 cm H2O per L/min error in ventilation per second being suitable, and AMIN and AMAX are limits set on the degree of support A for comfort and safety, 0.0 and 20.0 cm H2O being generally suitable. Unfortunately, while this method achieves a guaranteed minimum ventilation, there is little attempt to keep the support precisely in phase with the patient's own respiratory efforts. Rather, the system merely makes a step change in pressure at the start and end of inspiration.
In a more advanced servo-controlled ventilator, both guaranteed ventilation and phase synchronization is achieved. This apparatus is the subject of the commonly owned International Patent Application entitled “Assisted Ventilation to Match Patient Respiratory Need,” International Publication Number WO 98/12965 (hereinafter referred to as “AutoVPAP”). The AutoVPAP apparatus provides an instantaneous mask pressure P(t) based upon a substantial fraction of the patient's airway resistance R, respiratory airflow f(t), an amplitude A, and an estimation of the patient's instantaneous respiratory phase Φ as applied to a smooth pressure waveform template Π(Φ) as follows:
P(t) = P0 + R f(t) + A Π(Φ)for all f(t)(inspiration and expiration)where:A = G ∫ (V(t) − VTGT ) dtV(t) = 0.5 abs ( f(t) )AMIN < A < AMAX.In estimating the respiratory phase, the AutoVPAP apparatus uses a respiratory airflow signal and its derivative as input data for a set of fuzzy logic rules that are associated with particular phases of respiration. Using the results of the evaluations of the rules, a single phase value is derived and used as the instantaneous respiratory phase. Thus, the degree of ventilatory support is varied in phase with the patient's respiration. Moreover, as the calculation of A is based upon a target ventilation VTGT, a guaranteed level of ventilation is provided.
However, in this AutoVPAP system, room for improvement exists in the phase determination due to the problem of leak. Mask and/or mouth leak is ubiquitous during noninvasive ventilatory support using a mask, and is particularly problematical during sleep. Leak causes mis-measurement of the respiratory airflow, and therefore can severely interfere with patient-machine synchronization.